In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. To avoid overexposed or underexposed, exposure should be adjusted to control the amount of light from the subject. Nowadays, most cameras can be operated in an automatic exposure mode to automatically calculate and adjust exposure settings in order to properly capture images. The exposure settings include exposure time and illumination level, controlled by shutter speed and lens aperture, respectively. Slower shutter speeds (longer exposure time) and greater lens apertures (admitting more light) result in greater exposures.
Please refer to FIG. 1, a flowchart illustrating the conventional exposure control method. At the beginning, an image is captured (step 102). Then the system analyzes brightness of the entire image (step 104). If the analysis result is unsatisfactory or unacceptable, the system adjusts the exposure settings to be used for the next capture (step 106). For example, if the image is too bright, the system may shorten the shutter time or decrease the lens aperture or take both. Hence, the next photographing receives less light to make the next image a little dark.
Unfortunately, the brightness estimation is easily affected so as to possibly obtain inaccurate estimation. For example, when a certain volume of dark or bright object is passing the front of the photographic apparatus, the apparatus usually mistakes for change in light source and automatically adjusts the exposure settings. Please refer to FIG. 2 illustrating such case. In the image 200, a person 206 in a white T-shirt 208 and white trousers 210 is walking on a dark floor 204. A black bag 202 is placed on the floor 204. When the person 206 is moving close to the bag 202, the conventional exposure control method considers that the light source is changing. Hence, the exposure settings are adjusted during the moving of the person 206. In fact, the light source is unchanged. The inaccurate estimation and unnecessary adjustment incur an instable brightness, which adversely affects the quality of the photographing.
Therefore, there is a need of providing an improved exposure control method for automatically and properly adjusting the exposure settings during each photographing. It is desired that the moving of a dark or bright object is detectable and a proper control is made according to the detection.